Sitting on a bench at the University of Maryland is the first-ever desktop model of the Big Bang.

Don't worry, the 20-micrometer-wide device simulates how light behaved and time flowed at the universe's spark, not the explosion itself. It could someday help explain why time marches in only one direction.

"What we have done, with simple experimental geometry, is reconstruct the way that space-time expands," said Igor Smolyaninov, who describes the model in a paper submitted to Physical Review Letters.

Smolyaninov and Yu-Ju Hung, both electrical engineers at the University of Maryland, made their Big Bang simulation from exotic substances called metamaterials, which use alternating slices of different materials to twist light in unusual ways.

Researchers have suggested using metamaterials as invisibility cloaks that bend light around objects, to disguise one object as another and to build a perfect lens. A few years ago, physicists realized that metamaterials can also mimic astronomical events: a planet orbiting a star, light being trapped in black holes.

Building a toy cosmos in the lab lets physicists run otherwise impossible experiments on the nature of space and time. With Smolyaninov and Hung's setup, researchers could study the thermodynamic arrow of time, a long-standing problem in physics.

Most physical laws work just as well whether time runs forwards or backwards, but not the Second Law of Thermodynamics. It dictates that disorder must always increase with time. That's why people can't age backwards, eggs can't become uncracked, and Groundhog Day is fiction.

Another exception is the "cosmological arrow of time," which points forward from the Big Bang in the direction of the universe's expansion. This arrow could be linked to the thermodynamic arrow and point toward the ultimate heat death of the universe, but it could also someday reverse if the universe collapses in a "Big Crunch."

"While it is generally believed that the statistical and the cosmological arrows of time are connected, we cannot replay the Big Bang and prove this relationship in the experiment," the researchers write. But with a metamaterial Big Bang on the lab bench, they can.

To build their desktop Big Bang, the researchers arranged strips of acrylic and gold so that laser light hitting the gold excites waves of free electrons called plasmons. The math describing their path through the metamaterials' flat surface is the same as math describing how massive particles move through a flat version of Minkowski space, which contains dimensions for both space and time.

Light paths through the metamaterial Big Bang thus represented the life of a particle plotted in space and time, what physicists call "world lines." When the researchers illuminated the metamaterial with green laser light, they saw a triangle of plasmons extending from a single point - the "toy Big Bang" followed by an expanding universe's worth of world lines.

Because the metamaterial is imperfect, the light rays are distorted and interact with each other as they spread. This serves as a crude model of entropy, Smolyaninov says, representing the thermodynamic arrow of time. The world lines modeled the cosmological arrow. In the metamaterial, both arrows flowed in the same direction.

"It's reasonably good and interesting model of time," Smolyaninov said.

The researchers also considered whether their model allows time travel. At first, Smolyaninov thought this should be easy: If you built a metamaterial in which light moving in a circle was mathematically identical to particles moving through time, then sending a plasmon on a circular orbit should bring it back to the same timelike point it started at.

But it turns out to be more complicated. Only light with specific wavelengths and polarizations can work in Smolyaninov's space-time model. For light that behaves like a particle moving through time, circular orbits are impossible.

"Time travel in this model looks like it's impossible," said Smolyaninov.

Mechanical engineer Cheng Sun of Northwestern University isn't sure the analogy between the plasmons' paths and the expansion of the universe fully holds up.

"He tried to make a connection, and that's an interesting way of looking at the problem," Sun said. "But from the optical point of view, I have some reservations for the connection … I'm just not sure how much that will help you to understand the physical process."

Smolyaninov acknowledges that the model is limited. "It will never give you the real final answer about the real Big Bang and real time," he said. "But if you study it, you may discover something, and you may be able to ask more intelligent questions."